Every day, Apollo’s fiery chariot makes its way across the sky, bringing life-giving light to the planet. Ancient Greeks and Romans worshipped Apollo as a god of medicine and healing, as well as sun and light, but he could also bring sickness. Researchers today agree that exposure to ultraviolet radiation (UVR) in sunlight may have both beneficial and harmful effects on human health.
Over the past century, most public health messages have focused on the dangers of too much sun exposure. Indirectly, UVA radiation (95–97% of the UVR reaching Earth’s surface) can contribute to skin cancer by generating DNA-damaging molecules such as hydroxyl and oxygen radicals. UVB radiation causes sunburn; it can also cause DNA damage and promote skin cancers. The two types of radiation damage collagen fibres, destroy vitamin A in the skin, accelerate ageing of the skin, and increase skin cancer risk. Getting too much sun can also cause cataracts and diseases that are exacerbated by UVR-induced immunosuppression, such as reactivation of some latent viruses.
According to the 2006 World Health Organisation (WHO) report The Global Burden of Disease Due to Ultraviolet Radiation, excessive UVR exposure accounts for less than 0.1% of the total global burden of disease. DALYs indicate how much a premature death or disability reduces a person’s life expectancy. In her article, coauthor Robyn Lucas, an epidemiologist at the National Centre for Epidemiology and Population Health in Canberra, Australia, explains that many diseases linked to excessive UVR exposure tend to be benign, except for malignant melanoma, and they tend to occur in older age groups, mostly because of the long lag between exposure and manifestation, cumulative exposures, or both. As a result, when measured by DALYs, these diseases have a relatively low disease burden despite their high prevalence.
In contrast, the same WHO report noted that very low levels of UVR exposure may result in a markedly higher disease burden of 3.3 billion DALYs worldwide. There is an increased risk of various autoimmune diseases and life-threatening cancers associated with this burden, which includes major disorders of the musculoskeletal system. One of the best-known benefits of sunlight is its ability to increase the body’s vitamin D supply; vitamin D deficiency is most often caused by a lack of outdoor sun exposure. 1,25-dihydroxyvitamin D3 (1,25[OH]D), the active form of the vitamin, is now thought to regulate 1,000 genes, including several involved in calcium metabolism and neuromuscular and immune system functions.
Besides vitamin D-related health benefits of sun exposure, there may be other health benefits not acknowledged in the debate over how much sun is needed for goodSun-Dependent There There may be other health benefits to sun exposure that have gone largely overlooked in the debate over how much sun is needed for good health [see “Other Sun-Dependent Pathways,”athways,” p. A165]. As for what constitutes “excessive” UVR exposure, there is no one-There may be other health benefits to sun exposure that have gone largely overlooked in the debate over how much sun is needed for good health [see “Other Sun-Dependent Pathways,”